Cap for a writing instrument



FIG. 1 is a rear elevational view of a cap for a writing instrument showing my new design;

FIG. 2 is a front elevational view;

FIG. 3 is a right side elevational view, the left side being a mirror image thereof;

FIG. 4 is a top plan view;

FIG. 5 is a bottom plan view; and,

FIG. 6 is a top, left front perspective view thereof. 

The ornamental design for a cap for a writing instrument, as shown and described. 